In my experience trying to sell people regulators, most can't tell the difference in breathing between the bottom of the line regulator and the top of the line $1500 regulator while in the shop. This makes it hard to convince someone to spend another hundred or more on a regulator that really is better. When breathing a regulator at the surface, generally they won't perform as well as they do even in just a few feet of water. A side by side comparison on the surface tells one very little how either of them will perform at depth. This is why the magazines test breathing performance on a several thousand dollar machine.
Secondly, in the all Atomic second stages, they have what they call an AFC. It serves the same purpose as the venturi switch you see on many regulators now, which is to prevent a freeflowing regulator on the surface. However it is done automatically in the Atomics (hence the absence of a switch) and doesn't open up until you reach a certain depth which I believe is about 15 feet. So breathing an Atomic hooked up to a tank in the shop, you can expect that it will do better at depth.
OK, before I go too far off on a tangent, I can assure you that the Z2 will breathe just as well, in any condition as the T2 which cost around 4x more. In addition to this, The Atomic line of regulators consistently tops the charts as the best breathers on the market. Personally, I would buy the Z2.